Thank you for your asking. Because you are now in Android 10 V424, kindly refer to the steps below
WW-17.2018.2004.424>SD card downgrade with MaxProM2_downgrade_to_9>WW-16.2017.1912.072>auto upgrade to your referred firmware version
You may follow the steps below as 10 to 9 downgrade steps.
1. Download " MaxProM2_downgrade_to_9" and then put the image file into the root directory of the MicroSD card.https://www.asus.com/Phone/ZenFone-Max-Pro-M2/HelpDesk_BIOS/
2. Insert MicroSD card into phone and then power off the device.
3. Long press with "Power" key and "Volume Down" key together and stop after feel the vibration to enter Recovery Mode
4. Use "Volume Down/Up" key and press "Power" key to choose "Apply update from external storage"
5. Select the downgrade file
6. When the updating process is done, Choose "Wipe data/factory reset" and tap"Yes" to perform the master clear.
7. "Wipe cache partition" > "Yes"
8. "Reboot system now"
#Android 10 to Android 9 Guide https://www.asus.com/support/FAQ/1042349